Canadians like to point out all the things they have that America doesn’t: Mounties, the metric system, universal health care. But you might not know that there’s one thing they’re a little short on north of the border: gravity. That’s right: Canada actually has less gravity than it’s supposed to. The reasons for the shortage have puzzled scientists for decades.

Gravity isn’t uniform all over the Earth’s surface. It’s a result of mass, which means the varying density of the Earth at different locations can affect how much you weigh there. Canadians aren’t all free-floating like Sandra Bullock, but the effect is definitely measurable. In the Hudson Bay region, the average resident weighs about a tenth of an ounce less than they would weigh elsewhere. (And much less than they’d weigh if they moved south, where there are more Tim Hortons. But that’s a different story.)

Satellite data collected by GRACE—the Gravity Recovery and Climate Experiment—has recently solved this mystery. During the last ice age, Canada was covered by a vast glacier called the Laurentide Ice Sheet. This sheet was two miles thick over northern Quebec and stretched as far south as modern-day New York and Chicago.

Ice is heavy, so five million square miles of it pushed down on the rock underneath, squishing it like a Nerf ball. When the ice began to melt, about 21,000 years ago, the Earth began to spring back, but, like a Nerf ball, it takes a while. To this day, the Earth in the Hudson Bay region is still deformed, with lots of rock-mass having been pushed outward by all the ice. Less mass means less gravity.

The Laurentide Ice Sheet hasn’t quite melted all the way. The Barnes Ice Cap, in the middle of Baffin Island in northern Canada, is all that remains of a sheet that once covered hundreds of thousands of square miles. But the last of the Laurentide ice may not be around for long. In 2008, scientists found that global warming was speeding up its melt rate by a factor of ten. If you want a margarita made from 20,000-year-old ice, you better get up there quick.